1. Field of Invention
This invention relates, in general, to a treatment plant for the recovery of metal from hazardous waste. More particularly, the invention relates to a sequential process for recovery of precious metal from a waste solution in two stages involving first the electrolytic recovery of the greater portion of the metal from the hazardous waste material, followed by further recovery of metal in the second stage by means of ion exchange or shredded steel wool, and apparatus suitable for use in that two stage process. Still more specifically, the invention relates to a process and apparatus for recovering silver metal from spent photographic fix solution, first electrolytically, followed by further recovery using ion exchange or shredded steel wool.
2. Description of the Prior Art
Black and white photographic film is a highly light-sensitive photographic material which is sold commercially in the form of roll films, plates or photographic papers. It is used for black and white photography.
This film, in general, comprises a light-sensitive emulsion layer on a substrate such as a layer of cellulose acetate. The emulsion comprises silver bromide crystals in a binder of gelatin as the main components; however, the silver bromide emulsion is not equally sensitive to all colors of light. For this reason the coating is chemically sensitised by the addition of minute quantities of gold, mercury, and other heavy metal ions to the silver bromide and by feeble reaction with sulphide ions. Also, the coating is physically sensitised by the addition of pigmental sensitisers and thus made sensitive to a wider range of hues.
The photographic image is formed by the light rays which come from the subject being photographed and are focused on the light-sensitive coating of the film by the lens system of the camera. Each ray of light that strikes the film encounters silver bromide crystals, which are thereby activated. The exposed film is then developed in a development bath, which is a solution of a chemical reducing agent (developer). The latter reduces the activated silver bromide particles to black metallic silver. Silver bromide which has not been activated by exposure to light is not reduced by the developer and can be dissolved out of the emulsion coating by immersion of the developed film in a so-called "fixing bath", which contains ammonium thiosulate or sodium thiosulfate ("hypo"), sometimes incorrectly called "sodium hyposulfite", that dissolves unexposed silver bromide.
After the developer and fixing solutions have been washed out of the film and the latter has dried, the result obtained is the negative. On a negative the shadows appear as "white" (transparent) areas, while the highlights appear black. If the negative is then placed in contact with photographic paper (positive paper) and exposed to light, then the areas of this paper under the (dark) highlights in the negative will receive little light, while the areas under the (white) shadows in the negative will be strongly exposed. On developing this photographic paper, a "print" is obtained in which the exposed areas come out dark, while the unexposed areas remain white, and of course those areas which are somewhere between deep shadow and bright highlight come out in various intermediate shades of grey. The dark and light areas of the negative are thus reversed in the printinq process.
It is well known that the effectiveness of a hypo solution is reduced as the silver content thereof is increased. Thus, it is common practice to recover the silver in expended "hypo" photographic fixing ("photo fix") solutions by one of three prevalently used methods: (1) chemical precipitation, (2) metallic replacement (ion displacement), or (3) electrolytic plating. The latter method, in general, consists of flowing a direct electric current through the spent "hypo (or photo fix)" solution, between electrodes which causes the silver to be deposited in a plate form on the negative electrode (the "cathode").
The electrolytic plating method is generally conceded to be the most desirable of the three methods, since if the current densities are controlled and adjusted to conform to the varying concentrations of silver in the spent "photo fix" solution, the efficiency of silver recovery can be greater than with the other methods, and, when the concentration of silver in the spent "photo fix" solution has been lowered substantially, permits the recycling and re-use of the solution, if desired, thereby permitting considerable monetary savings in the purchase of photographic chemical solutions.
The chemistry of the electrolytic separation is somewhat complex. It is believed that the deposition of silver at the cathode is a secondary reaction. The hypo salts yield sodium or ammonium cations and thiosulfate anions. The thiosulfate ions which are negatively charged migrate towards the anode and upon reaching the anode are oxidized and the free silver ions remain in the solution. At the cathode, the sodium cations are discharged as sodium atoms and are so reactive that they decompose any ions which exist in the vincinity of the cathodes. If any thiosulfate ions are present, silver is deposited on the cathode plate (negative electrode). If not, highly reactive sodium ions reduce the hypo to a series of compounds, most of them fatal to the hypo solution. One of the compounds which is generally present when an insufficient amount of thiosulfate ions are present in the vicinity of the cathode is silver sulfide. The silver sulfide is dark brown in color and shortens the life of the hypo solution. Thus, in order to avoid the formation of silver sulfide in the solution, an adequate supply of thiosulfate ions in the vicinity of the cathode is believed a necessity. Nevertheless, this is a particular problem. Thiosulfate ions are not necessarily uniformly dispersed throughout the spent solution, particularly in a stagnant solution. And, as the thiosulfate ions are located in the solution at different distances remote from the anode, the migration toward the anode varies somewhat. Moreover, the concentration of silver ions in the photo fix solution is constantly changing with progression of the electroplating of silver from the solution. And, this concentration is not uniform throughout the solution, particularly in the vicinity of the cathode when silver has just been plated out of solution.
Furthermore, as the silver concentration is reduced in the solution, as a result of silver being plated out on the cathode, the power requirement across the electrodes changes. Thus, this power requirement must be related to the concentration of the silver in solution. If too much power (hence current too great) is applied to the electrodes with respect to the amount of silver available in the solution, a chemical reaction occurs within the solution which produces a sulfide gas (a process which is sometimes referred to as "sulfiding"). The generation of sulfide gas creates a very strong order in the environment and, if permitted to continue, will cause the silver plated out to burn and flake, permitting flakes of silver sulfide to wash away with the wash solution. On the other hand, if the voltage applied to the recovery system is too small (and therefore insufficient current) with respect to the quantity of silver in the solution, then silver in the spent photo fix solution may not be recovered in the time permitted for recovery. As a result, not only will dissolved silver be lost in the waste but, most importantly, as hereinafter more fully disclosed, the waste may contain environmentally unacceptable levels of silver. Thus, it is desired to operate at voltages whereby to recover from the spent photo fix solution the maximum amount of silver in the shortest period of time possible, and without sulfiding, but also to reduce the silver concentration to environmentally acceptable levels.
Over the years various and numerous devices have been invented to accomplish the recovery of silver from photo fix solution, and more efficiently, to better control the current densities during the electroplating operation, and to permit more rapid and uniform plating of silver from the photo fix solution. None of the systems, however, with which I am acquainted are entirely successful. Exemplary of this prior art are U.S. Pat. Nos. 2,791,555; 3,551,318; 3,560,366; 3 715 291; 3,964,990; 4,257,864; 4,305,805; and 4,652,352.
U.S. Pat. No. 2,791,555 discloses a method and apparatus for rejuvenating and prolonging the life of hypo solutions. The apparatus therein disclosed comprises a cathode element comprising a plurality of horizontally disposed circular shaped plates, in spaced-apart location on a vertically disposed, rotable shaft. On each side of the cathode element in diametric opposition to one another are located vertically disposed anode plates, connected together at their top by a conductive strip. The patentee discloses that such a rotatable cathode causes a shearing action in the hypo solution, resulting in there always being available at the surface of the cathode plates a sufficient amount of thiosulfate ions to permit appropriate plating of the silver without formation of silver sulfide. Nevertheless, the cathode plates in the invention disclosed in U.S. Pat. No. 2,791,555 are horizontally disposed while the anode plates, rather than being in a face-to-face relationship therewith, are located perpendicular to the cathode plates, in vertical disposition.
U.S. Pat. No. 3,551,318 discloses apparatus like that in the previously mentioned patent, i.e., U.S. Pat. No. 2,791,555. Nevertheless, the patentee discloses a system in combination therewith whereby the plating current is turned off gradually as a critical silver concentration is approached in the spent photo fix solution. Thus, plating is not allowed to occur beyond a critical concentration level of silver in the solution, thereby avoiding, according to the patentees, any undesirable side reactions due as a result of excessive power supplied to the electrodes. Like the anode and cathode plates in U.S. Pat. No. 2,791,555, the electrodes disclosed in this patent are oriented perpendicular with respect to one another resulting in somewhat less efficient power requirements than if the electrode plates were arranged face-to-face. Moreover, it has been determined that with horizontally disposed circular-shaped rotating cathodes such are not plated uniformly across their surface, a heavy concentration of silver plates at the outer periphery.
U.S. Pat. No. 3,560,366 discloses a uniquely designed vertically disposed cathode in the form of a cylinder or polyhedron mounted for rotation about a vertical axis. The anode comprises four vertically disposed cylindrical-shaped members in close proximity of the cathode. Thus, according to the patentee, a controlled interchange of the photo fix solution across the cathode surface is obtained.
U.S. Pat. No. 3,715,291 discloses silver recovery means for photo fix solutions wherein there is provided, in combination with the electrolytic silver recovery unit, an auxiliary tank upstream thereof to receive the silver waste solution from film processing equipment or other sources. A metering pump delivers the waste solution from the auxiliary tank to the silver recovery unit, the pump being started and stopped in dependence upon changes in the level of the waste solution in the auxiliary tank. The silver recovery unit operates only when the metering pump is running. Thus, according to the patentee, as a constant flow of solution enters the electrolytic unit, and at a constant silver concentration, this enables the silver recovery unit to be operated at a constant current setting selected appropriate to the particular silver concentration in the spent photo fix solution in the auxiliary tank. That being the case, according to the patentee, the problems associated with varying silver concentrations and a more or less arbitrarily selected current value disappear. Nevertheless, one problem with a spent photo fix solution is its excessive corrosiveness. As a result, such a solution would readily attack, it is believed, the sealed joints where the suction pipe connects the holdup tank to the pump and the sealed joints whereby the delivery pipe connects the pump to the holdup tank and to the silver recovery unit. Thus, the maintenance connected with such apparutus and the continual replacement of seals makes for not only somewhat unsatisfactory operation but added costs as well.
U.S. Pat. No. 3,964,990, like U.S. Pat. No. 3,715,291, discloses a holding tank for spent photo fix solution in combination with an electrolytic unit. The spent solution, according to this invention, however, is pumped into the recovery unit intermittently in dependence upon the concentration of silver in solution in the recovery unit. The system operates with a substantially constant voltage across the electrode plate in the recovery unit, with the concentration of the solution in the recovery unit being maintained substantially constant by the intermttent operation of the pump submerged in the holdup tank. The anodes and cathodes are located in spaced apart position, in horizontal disposition, on a vertically disposed rotable shaft, and are of octagonal shape. Round plates, however, according to the patentee are not desired in a system of this type, since when a round cathode is used, silver tends to build up on the outer edge of the plate, interfering with the circulation of liquid around the plate and applying substantial forces to the interior plate-supporting structure. Nevertheless, with such an arrangement as disclosed, the corrosive nature of the spent solution presents problems to the operation of the submerged pump and the operation of the float valves.
U.S. Pat. No. 4,257,864 discloses a portable unit for recovering silver from photographic processing solutions which includes a cathode assembly supported for rotation about a horizontally disposed axis. A plurality of circular-shaped spaced-apart, plate-like thin discs are located along the shaft and define cathode elements on which silver is plated. These cathode plates and shaft are located within an an apertured, cylindrical-shaped barrel and rotate in combination therewith. Spaced apart, elongate, stationary anodes are located in parallel relation to one another and in the recovery unit below the rotatable cathode assembly. Thus, the electrode arrangement results in greater power requirements and less efficient overall operation and cost.
U.S. Pat. No. 4,305,805 discloses the combination of a silver recovery apparatus, a silver solution source, and a holding tank. The solution accumulates in the holding tank to a given quantity. When there is sufficient silver solution in the holding tank, the solution is then caused to flow to the silver recovery apparatus through a flow metering valve, and the silver recovery apparatus then operates to recover the silver. The holding tank disclosed, however, suffers from the problem that spent photo fix solution is discharged from near the bottom thereof. As a result, the corrosive nature of the solution attacks the seals at the outlet opening and those of the valves provided in the conduit connecting the holding tank with the silver recovery unit.
U.S. Pat. No. 4,652,352 discloses recovering metals from dilute solution utilizing ion exchange and, optionally, electrolytic recovery. Thus, according to the patentee metals may be recovered from spent electroplating rinse solution for reuse in the electroplating both with essentially no generation of waste.
Of particular concern, as above-mentioned, with apparatus used in the recovery of silver from spent photo fix solutions is the corrosive nature of such solutions. In relatively short time bearings and the like exposed to such solutions become corroded and need be replaced. Seals degenerate, resulting in leakage, and needed replacement. Both add to the cost of such recovery operations not only because of the cost of replacement of equipment and parts, but also in down time and need for maintenance personnel. Moreover, because of the corrosive nature of these solutions it has not been found possible to entirely prevent leakage of the solution through bearings and fittings in contact therewith. As a result, the spent photo fix solution often leaks out of holding tanks and electrolytic units now being used, into the adjacent work area, creating not only a messy area and a housekeeping problem, but an unsafe condition, in the event that an employee's skin comes into contact with such leaked solution.
Although there has been concern, particularly over the past several years, about the disposal of hazardous waste-materials, the regulations of the Environmental Protection Agency (EPA) have exempted, until somewhat recently, precious metal wastes. Nevertheless, such waste material has been subject to local air and sewage discharge regulations. As a result of these local regulations, it has become somewhat common practice for many of the smaller photographic film developers, in particular, to collect such "hypo" or "photo fix" waste materials and to have that waste, before disposal, treated by independent contractors. Thus, it has been common practice for some years now for "recycling companies" to pick up the photographic fix waste material from photofinishers, printers, hospitals, and other developers of photographic film and x-rays, etc. and to recover the silver from that waste material, making the photo fix solution suitable for disposal or, in some cases, reusable by the film developer.
Now, however, with new regulations established by the EPA, and beginning in 1986 precious metal hazardous waste materials are no longer exempt from its regulation. Moreover, the EPA has set the standards by which the local water districts must comply. In the case of silver dissolved in photo fix solutions that requirement is now less than 5.0 ppm. Thus, the developers of all photographic film, x-rays, etc., and the independent contractors recovering silver from spent photo fix solutions, or those involved in recycling the solution, must comply with these new standards. Nevertheless, with such waste material no longer being exempt from the EPA regulations, and with other increased costs of operation in meeting not only the EPA but also local waste discharge requirements, it may no longer be economically feasible for these independent contractors and recycling companies to continue in operation, and to perform the services for the photographic and x-ray film developers that have been performed in the past. That being the case, it may be that each such a film developer will have to perform its own silver recovery operation on the photo fix waste, in order to be able to dispose of the waste material, or to reduce the silver concentration therein so that it can be reused. Of concern, however, even to the photographic and x-ray film developers, particularly those more localized and having relatively small operations, is the cost of an electrolytic recovery unit that will reduce the silver content in the spent photo fix waste material to meet the EPA standards, whereby such waste material can be discharged to waste streams, or otherwise disposed of. To my knowledge, there is now no commercially available unit for less than about $15,000.00 that will provide adequate recovery of silver from the photo fix waste material, and reduce the silver concentrations in that material to the extent required by the EPA.